Azonium group

Deprotonation from the azonium group leaves a lone pair of electrons on the nitrogen atom, and a neutral aza substituent. The known parent monocyclic azines (see Scheme la) include all the possible diazines and triazines, but only one tetrazine, the 1,2,4,5-isomer. Some 1,2,3,5-tetrazines have been reported, but only when heavily substituted or fused. Some aromatic bicyclic 1,2,3,4-tetrazines have been prepared (see Section 4.4.8.2.3) as well as reduced 1,2,3,4-tetrazines (see CHEC 2.21). No pentazines are known. All attempts to prepare hexazine also failed though several claims about fixation of the latter in a matrix have appeared. 

A remarkable case of a Meerwein reaction of phenylacetylene was reported by Leardini et al. (1985) in a new synthetic route to benzothiophene derivatives. Aryldi-azonium salts with a thioether group in the 2-position add phenylacetylene and substituted phenylacetylenes in the presence of metallic copper or iodide ion in acetone, or of FeS04 in DMSO (Scheme 10-60). The radical 10.21 formed initially is attacked intramolecularly by the sulfur atom of the thioether group to give the benzothiophene 10.22 in high yields (55-95%) as shown in (Scheme 10-60). Lear-... 

There are apparent exceptions to the rule that aromatic compounds are azo coupling components only if they contain a hydroxy or an amino group. A long time ago Meyer and Tochtermann (1921) demonstrated that the 2,4,6-trinitrobenzenedi-azonium salt couples with mesitylene, isodurene (1,2,3,5-tetramethylbenzene), and pentamethylbenzene (see also Smith and Paden, 1934). That result was surprising at the time, but today it is, of course, understandable the diazonium salt used is pro-... 

Tyrosine may be targeted specifically for modification through its phenolate anion by acylation, through electrophilic reactions such as the addition of iodine or di-azonium ions, and by Mannich condensation reactions. The electrophilic substitution reactions on tyrosine s ring all occur at the ortho position to the —OH group (Fig. 11). Most of these reactions proceed effectively only when tyrosine s ring is ionized to the phenolate anion form. 

Laali and coworkers132 investigated the reaction of the gaseous 2-f-butyl-3-phenyl-phosphirenylium ion (a 2jr-Huckel phosphirenylium ion) with aniline and other nucleophiles and dienes via MS" experiments. In ion/molecule reactions, the ion reacts readily with aniline to form an azonium ion via nucleophilic attack at phosphorus by the amino group, as suggested by calculations and MS3 experiments. 

Substitution at C(sp ) normally occurs by an addition-elimination mechanism under acidic conditions. Electrophiles add to nucleophilic C=C tt bonds, including most arenes, and nucleophiles add to C=N, C==0, and electrophilic C=C TT bonds. Arylamines can undergo substitution of the NH2 group by a di-azonium ion mechanism. 

The hydrazins are theoretically derivable from the group BaN—NHa, diamidogen, by the substitution of acid, alcoholic, or phenylic radicals for one or more of the hydrogen atoms. They may be primary, secondary, tertiary, or quaternary, as the sub-.stitution removes are two, three, or four of the H atoms. The secondary hydrazins may be symmetrical or unsymmetrical, according to the formula RHN—NHR and RaN—NHa. They also form compounds resembling the ammonium salts, known as azonium -compounds. In short, they play the part of compound ammonias. The substituted groups may be acyclic as in ethyl-hydrazin, CaHsHN —NHa, but in the inajority it is a cyclic derivative as in phenyl hydrazin. 

Zollinger s group (Szele et al., 1983 a) studied the reactions of such an alkenedi-azonium salt with O-nucleophiles (ethers, alcohols, water, 2-naphthol) and with a secondary amine (diethylamine), since, in the latter reaction, the cyclization of Scheme 9-46 would not be possible. 

It is well known that azobenzenes containing amino or dialkylamino groups undergo a pronounced color change in solution or in polymer film under protonation [45]. This phenomenon, called halochromism, is the basis of their use as optical sensors and pH indicators [46].The relative effect of the color changes in such dyes is associated with the existing ammonium-azonium (bt a) tautomerism and the tautomeric ratio, which depends, as usual, on the solvent, temperature, and existence of additional substituents in the protonated molecule. 

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